1. Field of the Invention
The present application generally relates to a thermal conductivity detector comprising at least four detector components arranged in receptacles of a thermal conduction block in a circle around a center axis of the thermal conduction block.
2. Related Art
Thermal conductivity detectors, such as those disclosed in U.S. Pat. No. 2,512,857, U.S. Pat. No. 3,474,660 or DE 103 18 450 B3, are used to detect certain liquid or gaseous substances (fluids) on the basis of their characteristic thermal conductivity, particularly in gas chromatography. To that end, the substances to be detected are successively guided past an electrically heated filament disposed in a channel, after their chromatographic separation. Depending on the thermal conductivity of the substance flowing past, more or less heat is diverted from the heating filament to the channel wall, and the heating filament is correspondingly cooled to a greater or lesser degree. As a result of the cooling of the heating filament, its electrical resistance changes, which can be detected. For this purpose, the heating filament is typically placed in a measuring bridge, which contains additional resistors and an additional heating filament in a further channel through which a reference fluid flows. Instead of the resistors, further filaments may be provided that are fluidically parallel or in series with the filaments in the measurement channel and the reference channel, respectively. In the latter, the heating filaments and the surrounding channels are referred to individually as detector components, and collectively as the thermal conductivity detector.
To keep the detector components on the same temperature level, it is known from the above-mentioned references to have the detector components accommodated in a thermal conduction block that is made of a suitable heat conducting material such as brass or aluminum. For the same reason, the design of the thermal conductivity detector is thermally symmetrical with the detector components being arranged in a circle around a center axis of the thermal conduction block.
The detection limit of the thermal conductivity detector may be limited by thermal crosstalk between the detector components. For example, when a chromatographically separated gas fraction with a high thermal conductivity flows past the heating filament in the measurement channel and the carrier gas passing the heating filament in the reference channel has a lower thermal conductivity, the heat flows from the filaments to the respective channel walls. Accordingly, the wall temperatures will be different. Having different wall temperatures creates a temporary temperature imbalance within the thermal conduction block that affects the detector components differently. The thermal symmetry of the known thermal conductivity detectors is not good enough to compensate for this effect.